The invention relates to a method and an apparatus for the examination of specimen with a beam of charged particles. In particular, this invention relates to an objective lens and a method for focusing a beam of charged particles onto a specimen.
Charged particles e.g. electrons, negatively or positively charged ions coming from a particle source can be accelerated and shaped into a beam by applying electric and magnetic fields. Different electro-optical devices are used to guide the beam and finally to focus it onto a specimen or sample. When a particle of the incident beam strikes the surface of the specimen, it undergoes a series of complex interactions with the nuclei and electrons or the atoms of the specimen. The interactions produce a variety of secondary products, such as electrons, X-rays, heat and light. Many of these secondary products are used to produce images of the specimen, e.g. its surface structure, and to collect additional data from the specimen.
Commonly, an objective lens focuses the beam on the specimen. The objective lens cannot produce an infinitely small spot size due to lens aberrations but instead produces a narrowed spot which represents a demagnified image of the beam source. The diameter of the spot can be controlled, for example, by reducing the focal length of the objective lens which results in a spot of smaller diameter on the specimen. In general, the smaller the spot size, the better the resolution. It is becomes increasingly difficult to resolve any two structures whose spacing is less than the diameter of the beam of charged particles. Thus, the spot size is crucial for image quality and resolving power.
Two lens defects, spherical and chromatic aberration, are mainly responsible for the minimum spot size that can be reached and thus the resolution of devices known in the art. Spherical aberration causes electrons with a trajectory further from the optical axis to be deflected stronger than electrons with a trajectory closer to the optical axis. This results in an enlargement of the spot size. Chromatic aberration is the result of charged particles in the beam having different velocities and thus different wavelengths. The differing wavelengths cause the charged particles to be brought to focus at different points along the optical axis, which blurs the image. Both of these lens defects depend on the focal length of the objective lenses i.e. the distance between the principal plane of the lens and the specimen to be examined. A shorter focal length and therefore smaller lens defects can be obtained, for example, by an overall reduction of the lens dimensions.
In magnetic lenses, a short focal length is achieved by concentrating the magnetic flux generated by the excitation coils into a small area. At the same time, the pole piece gap and the center bore of the lens body are kept at minimal dimensions. A reduction of the geometrical dimensions of magnetic lenses to further reduce the focal length is, however, limited by the fact that the lens requires a given volume for the windings in order to obtain the necessary field strength. Furthermore, it is not possible to increase the current density in the excitation coils beyond certain limits due to the undesirable production of heat in the coils.
In electrostatic lenses, a short focal length can be obtained by increasing the strength of the electrical field. The occurrence of discharge effects, however, sets a limit to such an approach. Also, strong electrical fields prevailing on the specimen""s surface can result in imaging artifacts. An alternative way of reducing the focal length is to reduce the overall dimension of an electrostatic lens, in particular, the distance between the electrodes and the size of their apertures. Such a reduction can be achieved quite easily, however, the disadvantages entailed by this approach are the increasing difficulties in integrating elements like deflecting coils or stigmators.
In other electro-optical devices, magnetic and electrostatic lenses are combined into one objective lens. Frosien et al (EP 0 274 622 and 0 333 018) disclose a compound lens wherein the electrodes of the electrostatic lens are disposed within the magnetic lens. Compared with a simple magnetic lens, such a combination results in reduced aberrations. Adjustments of the focal length of such a lens result in variations of the electrical field prevailing on the surface of the specimen. These variations in turn cause deviations in the path of secondary electrons and thereby influence the image quality. In addition, the variations of the electrical field cause various effects due to charging of the insulating samples which influence the image quality as well. As optional preventive measures, additional shielding electrodes have been mounted between the compound lens and the specimen. Such additional shielding, however, limits the reduction of the focal length since it is positioned between the lens and the sample.
The present invention intends to provide an improved lens and method for focusing a charged particle beam on a specimen. According to one aspect of the present invention, there is provided an objective lens as specified in independent claim 1.
According to a further aspect of the present invention, there is provided a method as specified in independent claim 9.
Further advantages, features, aspects and details of the invention are evident from the dependent claims, the description and the accompanying drawings. The claims are intended to be understood as a first non-limiting approach of defining the invention in general terms.
According to a further aspect of the present invention, there is provided an objective lens for focusing a charged particle beam on a specimen. The objective lens comprises a first focusing lens positioned between a charged particle source and the specimen for finely focusing the beam of charged particles on the specimen. The objective lens further comprises a second focusing lens positioned between the first focusing lens and the specimen for coarsely focusing the beam of charged particles on the specimen.
The combination of a first finely focusing lens followed by a second coarsely focusing electrostatic lens into one objective lens allows the reduction of the distance between the principal plane of the objective lens and the specimen. This, in turn, results in a shorter focal length which entails a smaller probe diameter and a higher resolving power.
According to a still further aspect of the present invention, there is provided a first focusing magnetic lens followed by a second focusing electrostatic lens. This combination of two focusing lenses allows a considerable reduction of the focal length of the objective lens and achieves a high resolving power. Thereby, the magnetic lens has the advantage of being able to perform a fine focusing without influencing the field above the sample surface. In addition, due to its miniaturization, the electrostatic lens has the advantage of further reducing the focal length. This shifts the principal plane of the objective lens closer to the sample. The objective lens system can fully operate, and parameters like the primary energy of the charged particles can be selected freely. Further, there are no limitations with respect to scanning and correcting features if such features are used in combination with the objective lens.
In a preferred embodiment according to the invention, the second focusing electrostatic lens is miniaturized. Preferably, the first focusing magnetic lens comprises scanning and correcting devices in case such devices are used in particular applications.
According to still another aspect of the present invention, there is provided an objective lens for focusing a charged particle beam on a specimen. The objective lens comprises a first focusing lens positioned between a charged particle source and the specimen for finely focusing the beam of charged particles on the specimen; the first focusing lens being a magnetic lens. The objective lens further comprises a second focusing lens positioned between said first focusing lens and the specimen for coarsely focusing the beam of charged particles on the specimen; the second focusing lens being a miniaturized electrostatic lens comprising at least two electrodes.
The invention is also directed to methods by which the described apparatus operates. It includes method steps for carrying out every function of the apparatus. Furthermore, the invention is also directed to apparatus for carrying out the disclosed methods and including apparatus parts for performing each described method step. These method steps may be performed by way of hardware components, a computer programmed by appropriate software, by any combination of the two or in any other manner.